This application is a 371 of PCT/EP98/01816 filed Mar. 27, 1998.
The present invention relates to sintered silicon nitride (Si3N4), components made thereof, in particular valves, process for their production and their use.
Materials made of Si3N4 are of proven use in many applications. However, components made of Si3N4 still have inadequacies such as lack or reliability in continuous use, which stand in the way of wide use of Si3N4 components in the said applications, which would be advantageous economically and ecologically. For example, although DE-A 4312251 claims a high-strength Si3N4 material having a defined failure probability, no teaching is given regarding the way in which these failure probabilities are to be achieved. They are merely derived from classical flexural strength determinations and statistical evaluation thereof.
The reliability of ceramic materials is determined from short-term strength, with its spread, and from long-term behaviour under load. In this context, the short-term strength follows the Griffith relationship:                     σ        =                              K            Ic                                              c                        ⁢            Y                                              (        1        )            
with:
"sgr" Strength in MPa
Klc Fracture toughness in MPaxc2x7mxc2xd
c Critical crack length in xcexcm and
Y Form factor, which describes the shape of the critical crack.
According to this relationship, the strength is directly dependent on the crack or defect length in the material.
The scatter in the short-term strength, which is important for reliability, is described by the Weibull distribution and is characterized by the Weibull modulus according to DIN 51110.
In the case of loading below the stress which leads to catastrophic fracture, however, the xe2x80x9cv-kxe2x80x9d concept is applicable
v=Axc2x7Knlxe2x80x83xe2x80x83(2) 
with
v Crack growth rate in m/s,
Kl Stress intensity factor in MPaxc2x7mxc2xd in the case of load type l= tensile stress, and
A,n Parameters for subcritical crack growth (life).
This concept is applicable to materials and components which are exposed to varying stresses below the maximum stress, described by the Griffith relationship, which immediately leads to failure, and is therefore relevant to ceramic materials and components for a large number of technical applications, e.g. for valves in reciprocating piston engines.
The crack growth parameter is determined, according to the description of standard draft ENV 843-3 by determining flexural strengths at different loading rates.
The determination of the flexural strength is described in DIN 51 110. The loading rate employed in this case, which is intended to cause fracture in 5 to 10 s, is customarily about 100 MPa/s. The flexural strength determined in this case is referred to as short-term or inert strength "sgr"c.
In order to ascertain the crack growth parameter, this measurement is carried out at reduced loading rates. In this case, the cracks which are present have the opportunity to grow, with the result that fracture occurs under lower loads, i.e. so-called subcritical crack growth takes place. If the breaking stress is plotted against the loading rate on a double logarithmic scale, and the median values of the measurement carried out repeatedly for a defined loading rate are joined by a best fit line, then the crack growth parameters n and A are found from the slope of the line and the axis intercept of this line. Typical ceramic materials have n values of 30 to 40 (see Kingery, Introduction to Ceramics, John Wiley and Sons, New York, 1976, page 804) and are therefore apparently to be qualified as subcritical crack growth, so that their life in practical use is limited.
In order to satisfy increasing demands, especially in the automobile industry, a need has arisen for Si3N4 materials and components with improved reliability.
The object of the present invention was therefore to provide sintered Si3N4 and reliable components, in particular valves based on Si3N4, which have properties meeting this profile and are also straightforward and therefore inexpensive to produce.
It has unexpectedly been found that sintered Si3N4 with a particular chlorine content has improved subcritical crack growth behaviour with high flexural strength and high Weibull modulus at the same time.
The invention therefore relates to sintered Si3N4 which has a chlorine content of 100 to 500 ppm, a subcritical crack growth parameter nxe2x89xa750, preferably xe2x89xa760, a mean flexural strength at room temperature xe2x89xa7850 MPa and a Weibull modulus xe2x89xa718.
The chlorine content of the sintered Si3N4 was in this case determined by pressure digestion with hydrofluoric acid at temperatures between 100 and 120xc2x0 C. and subsequent potentiometric titration of the chloride by means of silver nitrate.
The sintered Si3N4 according to the invention preferably contains alkaline earth metals, Sc2O3, Y2O3, rare earth oxides, TiO2, ZrO2, HfO2, B2O3 and/or A12O3 as sintering additives, these forming a secondary phase concentration in the sintered material of 7.5 to 20 vol. % in addition to crystalline Si3N4 and/or Si3N4 mixed crystals.
This secondary phase concentration is determined by ascertaining the total oxygen content of the sintered Si3N4 through hot extraction. The known oxygen concentration introduced by the added sintering aids is subtracted from this result. The difference represents the oxygen content of Si3N4 following preparation, which is assumed to be present in the form of SiO2. This SiO2 concentration is added to the sintering aid concentration, which represents the total proportion of oxide constituents in addition to Si3N4.
For the Si3N4 proportion in the material, its pure density of 3.18 g/cm3 is employed to calculate the volume fraction, and for the secondary phases which are formed by the reaction of the sintering additives with the SiO2 in the Si3N4 powder during the sintering, the pure density xcfx81R is calculated according to                               ρ          R                =                  G          ⁢                      -                    ⁢                      tot            /                                          ∑                                  i                  =                  1                                                  i                  =                  n                                            ⁢                                                (                                                            G                      i                                        /                                          ρ                      Ri                                                        )                                ⁢                                  xe2x80x83                                ⁢                in                ⁢                                  xe2x80x83                                ⁢                                  g                  /                                      cm                    3                                                                                                          (        3        )            
with
G-tot=Total weight of the oxide components in g
Gi=Weights of the individual oxide components in g
xcfx81Ri=Pure densities of the individual oxide components in g/cm3.
The volume fractions of Si3N4 and secondary phase are thereby determined, the latter being between 7.5 and 20 vol. % for the material according to the invention.
The Si3N4 according to the invention is distinguished by a high packing factor (low porosity) so that, for example, during re-sintering at a temperature up to 50xc2x0 C. higher than the sintering temperature, neither the density nor the Young""s modulus of the material changes.
The invention also relates to a process for preparing the sintered Si3N4 according to the invention where Si3N4 powder, which either contains chlorine in an amount of 500 to 1500 ppm or, as an alternative to this, is used together with a metal chloride, is dispersed in water together with at lest one sintering additive, mixed with organic processing aids,
the aqueous slip is ground to a fineness of 90% less than 1 xcexcm,
and subsequently dried preferably by spray drying or fluidized bed drying so that the Si3N4 granules have a moisture content of between 1.0 and 4% by weight, preferably between 1 and 3% by weight and an average granule size of 40 to 80 xcexcm, and these are subsequently compressed and sintering is carried out after the organic process aids have been baked out under an N2 pressure of 1xe2x89xa6pxe2x89xa610 bar.
The compression is preferably carried out axially and/or isostatically.
In a preferred embodiment of the invention, the compression is carried out at pressures  less than 2500 bar, the organic process aids and the moisture are baked out in air, inert gas or vacuum at Txe2x89xa6650xc2x0 C. and the sintering is carried out under an N2 pressure of 1xe2x89xa6pxe2x89xa610 bar at Txe2x89xa62000xc2x0 C.
Preferably, the Si3N4 powder used has a Cl content of 500 to 1500 ppm and leads in the sintered Si3N4 to a Cl content of 100 to 500 ppm.
Preferred sintering additives which can be used in the process according to the invention are alkaline earth metals, Sc2O3, Y2O3, rare earth oxides, TiO2, Zro2, HfO2, B2O3 and/or Al2O3.
These are preferably added in amounts such that, during the sintering, by reaction with the oxygen which is always present in Si3N4 powders, and is assumed to be in the form of SiO2, a liquid phase is formed which is present in the sintered material as a predominantly vitreous secondary phase in a concentration of 7.5-20 vol. %.
Preferred organic process aids which can be used in the process according to the invention are dispersing agents and/or impression aids, the latter comprising the function of binding and plasticizing.
The dispersion agents are preferably citric and polyacrylic acid derivatives and amino alcohols in concentrations of 0.1-2.5% by weight in relation to the solids content of the slip.
A large number of substances can be used as compression aids, such as, for example, polyurethane dispersions, cellulose derivatives, starches and polysaccharides, polyamide solutions, polyvinyl alcohol and acetate, polyethylene glycols and/or stearates. These are preferably used in amounts of 0.2-5% by weight.
The compression at  less than 2500 bar may be carried out by axial and/or isostatic dry compression, for example in a mould corresponding to the component.
The materials obtained in this way have no granule residues in the material structure. Reproducible strengths in excess of 850 MPa with Weibull moduli xe2x89xa718 are therefore obtained.
The invention also relates to components, especially valves, made of the Si3N4 according to the invention.
The present invention furthermore also relates to valves with a failure probability of less than 10xe2x88x926 made of the sintered Si3N4 according to the invention.
The invention also relates to a process for producing the valves according to the invention, according to which the valves are selected using vibration analysis by frequency splitting of the resonant frequency peak xe2x89xa70.0125%. The other process steps are similar to the process according to the invention for the preparation of Si3N4, the compression being carried out in a mould corresponding to the valve to be produced.
It has now been found that, with the last-mentioned process, irrespective of the geometrical shape of the valve, defective components can be rapidly and definitively detected through frequency splitting of resonant frequency peaks.
This kind of selection is, however, also possible for components with a different geometrical shape.
In the process according to the invention, the valve to be tested is placed on the head side on three electrodynamic transducers, one transmitter and two receivers, in such a way that the valve is supported, at a distance of about 1-2 mm from the edge of the head, with an angular separation of the sensors equal to 120xc2x0 each. By varying the frequency of the transmitter in the 0.1 to 2 MHz range, flexural vibrations of the shaft and flexural vibrations of the head plate are stimulated in the ceramic valve, and are recorded using the two vibration receivers.
Macroscopic defects, such as cracks, inclusions of components extraneous to the material or nonuniformities in terms of the thickness and/or Young""s modulus are manifested by a clearly attributable resonance frequency split into 2 subsidiary natural vibrations, the frequency split increasing as the size of the fault increases.
The invention also relates to the use of the Si3N4 according to the invention and components produced therefrom in engine construction, especially as valves in reciprocating piston engines, in mechanical engineering in general, storage technology and in machine construction.
Engine parts produced therefrom are distinguished primarily by long life and high reliability. For example, Si3N4 engine valves on test rigs which were operated far beyond the conditions encountered in normal reciprocating piston engines, showed outstanding strength without failing.
The invention is further described in the following illustrative examples in which all parts and percentages are by weight unless otherwise indicated.